This invention relates to a water-absorbent resin of a new, novel type obtained from polymerization of water-soluble ethylenically unsaturated monomer and a process for producing this resin. In detail, it relates to a water-absorbent resin having average particle diameter in a specially defined range and a narrow range of particle distribution, being superior in water absorption capacity, water absorption rate, and suction force, showing that an amount of elution of water-soluble resin (hereinafter referred to as, water-soluble component) is only small, and being very suitable as sanitary materials, and a water-absorbent resin wherein the shape is not sphere angle-absent and also, being superior in handling and treating, and besides, a process for producing the water-absorbent resins by reverse-phase suspension polymerization under specified conditions.
Hitherto, an attempt has been carried out to use a water-absorbent resin as an absorbent sanitary material for absorbing body fluids such as a sanitary cotton, a disposable diaper, and the like. There have been known, as water-absorbent resins for this purpose, a hydrolyzed starch-acrylonitrile graft polymer (Japanese Official Patent Gazette, shouwa 49-43395), a neutralized starch-acrylic acid graft polymer (Japanese Official Patent Provisional Publication, shouwa 51-125468), a saponified vinyl acetate-acrylic acid ester copolymer (Japanese Official Patent Provisional Publication, shouwa 52-14689), a hydrolyzed acrylonitrile or acrylamide copolymer (Japanese Official Patent Gazette, Shouwa 53-15959), and crosslinked products of these polymers, a crosslinked product of a partially neutralized polyacrylic acid (Japanese Official Patent Provisional Publication, Shouwa 55-84304) and others.
As methods to produce a water-absorbent resin from a water-soluble ethylenically unsaturated monomer, although aqueous solution polymerization etc. are hitherto known in addition to the reverse-phase suspension polymerization wherein a monomer or its aqueous solution is suspended and polymerized, properties such as shape of resins obtained from these polymerization methods is almost settled depending on the respective process, some serious problems have been found in points of shape, handling properties, and safety etc. for water-absorbent resins obtained from previous polymerization methods. For instance, in a case of that a water-absorbent resin is produced by aqueous solution polymerization, since a powdering process is usually required after polymerization, the resin obtained is irregular, cracked particles having sharp angles and its particle distribution generally broad, so that a countermeasure for dust is necessary. Further, when reverse-phase suspension polymerization is performed, a resin obtained is of sphere or granular-type resulting from condensation of the round shape resin.
In this reverse-phase suspension polymerization process, a monomer or its aqueous solution is dispersed in a stable form in a hydrophobic organic solvent and polymerized, usually requiring a dispersing agent, so that this agent has a great influence on the decision of particle diameter and handling properties of water-absorbent resins obtained. As a dispersing agent being hitherto used in reverse-phase suspension polymerization, there have been known, for examples, nonionic surface active agents such as sorbitan fatty acid esters (Japanese Official Patent Gazette, shouwa 54-30710), glycerol or polyglycerol fatty acid esters (Japanese Official Patent Provisional Publication, shouwa 62-172006), sucrose fatty acid esters (Japanese Official Patent Provisional Publication, shouwa 57-167302) etc., cellulose derivatives such as cellulose ethers and cellulose esters (Japanese Official Patent Provisional Publication, shouwa 58-32607), and oleophilic polymers having carboxyl groups (Japanese Official Patent Provisional Publication, shouwa 57-21405). Among these, when the nonionic surface active agents are used, a polymer obtained has such a small mean particle diameter as below 100 .mu.m and the particle distribution becomes very broad, so that it is not enough in water absorption capacity and requires a countermeasure for dust when it handles in a powder condition, and accompanies formation of a large amount of an extraneous matter on the side wall of reaction vessels during polymerization. Also, a water-absorbent resin obtained has a small diameter and broad range of particle distribution, therfore, if a treatment solution is mixed to that with an improvement purpose, the mixing becomes ununiform and often, the improvement can not be performed uniformly. When the mixing becomes ununiform, a polymer powder makes in part a lump, which is hereinafter referred to as a fish-eye. When cellulose derivatives such as cellulose ethers and cellulose esters are used, the average particle diameter becomes large as about 100 .about.200 .mu.m, but there exists such weak points that these dispersing agents show in general low solubility for hydrophobic organic solvents, easily crystalize out at room temperature, are mixed in the finished product as a block form, and those once mixed in the finished product are again melted during drying and fuse together with the finished product. Further, when a oleophilic polymer having carboxy groups is used, there remains the dispersing agent on the surface of forming water-absorbent resin, so that the product is not safe enough in a case being employed as sanitary materials.
Besides, to obtain an improved water-absorbent resin being superior in electrolyte absorption capacity, having large particle diameter, and taking a rustling particle shape, there has been reported a method wherein to an aqueous solution of unsaturated monomer is added hydroxyethylcellulose and, by using a sorbitan fatty acid ester as a dispersing agent, reverse-phase suspension polymerization is performed (Japanese Official Patent Provisional Publication, shouwa 56-76419).
Thus, water-absorbent resins hitherto obtained from either aqueous solution polymerization or reverse-phase suspension polymerization all have had problems in points of shape, handling properties, and safety, as described above.
Also resin shape of this kind much affects the employing manner of resin. In general, a water-absorbent resin is frequently used in combination with other materials having absorption properties. For instance, in a case of that a water-absorbent resin is used as a water-absorbent agent in an absorbent body of sanitary materials such as a paper diaper or a sanitary cotton etc., there has been adapted a method wherein the water-absorbent resin being laminated among absorbent papers, being dispersed in a sandwich style among cotton-like pulps, or being blended with cotton-like pulps. If, in these cases is employed a water-absorbent resin of irregular and cracked shape, obtained by the above-described aqueous solution polymerization, having sharp angles and broad distribution of particles, there occurs a problem that an angle of resin having a large particle diameter tears a paper and therefrom, particles fall off or the resin is pressed out during swelling, for instance, in a case of that the resin is laminated among water absorption papers. Also, since a water-absorbent resin obtained from reverse-phase suspension polymerization is generally small in average particle diameter and of round shape, there occurs a problem in dispersion etc. when being laminated among the absorption papers, being dispersed in a sandwich style among cotton-like papers, or being blended with cotton-like pulps and, after distribution, there occurs also a problem that the resin very easily moves and drops out just before and after use of an absorbing body.
As described above, the resins being widely used at the present age are not enough suitable in size, particle distribution, and shape and, therfore, not sufficiently satisfactory in all of such points as water-absorbent capacity, handling and treating, properties for use and others.